I'm the author of the ASHFALL three-and-a-half-ology: ASHFALL, ASHEN WINTER, SUNRISE, and DARLA'S STORY.
Escape from the Past is a fascinating genre mash-up: the bread in this book sandwich is science fiction, while the meat of the novel is historical fiction. It's clearly young adult throughout, essentially a classic coming-of-age story for its teen protagonist, Max.
The science-fiction parts of the story may feel a little hand-wavy for some readers. I never quite understood how Max got transported to the past. I was able to read past my doubts, remembering Arthur Clarke's third law that "any sufficiently advanced technology is indistinguishable from magic."
The novel reads best as historical fiction--an adventure set in 1471 A.D. in what is today central Germany. The author, Annette Oppenlander, clearly knows the setting intimately and has thoroughly researched the area's history. Escape from the Past is chock-full of the tiny details that make a story feel realistic and immersive, from the leather ribbons used to fasten shoes to the slimy gruel that formed the bulk of the peasants' diet. I particularly enjoyed how Oppenlander shows the dramatic contrast between the way peasants and the aristocrats lived in the middle ages.
To sum up, I probably won't be recommending this book to teens who are primarily science fiction fans. But those who love historical fiction or medieval fantasy will certainly enjoy Escape from the Past.